Brake control mechanism



April 25, 1961 c.

BRISSON 2,981,377

BRAKE CONTROL MECHANISM Filed April 4, 1958 4 Sheets-Sheet l inunumgggmnnunv ATTORNEY April 25, 1961 L. c. BRISSON 2,981,377

BRAKE CONTROL MECHANISM Filed April 4, 1958 4 Sheets-Sheet 2 if (b I KWATTORNEY L. c. BRISSON 2,981,377

April 25, 1961 BRAKE CONTROL MECHANISM Filed April 4, 1958 J 4Sheets-Sheet 3 INVENTOR Zoo/s Gi /mass Bm/sso/v ATTORNEYS April 25, 1961Filed April 4, 1958 c. BRISSON 2,981,377

BRAKE CONTROL MECHANISM 4 Sheets-Sheet 4 JF- 58 66/ 67 g 66 5.9 54 72INVENTOR lows C'fl PLES ap/sso/v BY 2 z ATTORNEY S United States atent CBRAKE CONTROL MECHANISM Louis Charles Brisson, 8 Rue de la Duchesse deChartres, Oise, France Filed Apr. 4, 1958, Ser. No. 726,381

13 Claims. (Cl. 188--78) shoes which engage the drum on opposite sidesof the center of rotation behave differently under the reactionpressures of the moving drum on the shoes. In the ap plicants copending.application, Serial No. 651,477, filed AprilS, 1957, there is discloseda hydraulically'actuated brake system wherein the different reactionpressures on the two shoes generate shoe motions which are harnessed tofurther activate the hydraulic system in a manner which achieves highlyimproved efiiciency and regularity of action over a wide range ofoperating conditions. The invention of this copending application isparticularly useful when self-locking conditions obtain in the brakesystem because it is able to supply a force to the primary shoe whicheffectively overcomes the self-lock, thereby rendering the brake systemfully controllable at all times. The present'invention has as one of itsobjects to achieve many of the improved functions of that hydraulicsystem utilizing, however, mechanical linkages to accommodate andtransmit the heavier stresses attendant the braking process, therebyobviating the consequences of failure of the relatively less durablehydraulic components.

Another object of the invention is to provide a mechanically compensatedbrake system which achieves highly regular braking action. 7

Another object of the invention is to provide brake mechanism which isat least partially mechanical in nature and which is capable of fullycontrolling a selflocking brake action to achieve a brake system ofconsistently high efficiency.

Another object of the invention is to provide a brake system which isrobust in construction and both simple and inexpensive to manufacture.

Still another object of the invention is to provide a brake system whichis free of the normal shortcomings of hydraulic systems of the samegeneral type including, for example, freedom from theproblems ofhigh-pressure hydraulic-seal leakage and high-pressure seal friction.

Briefly stated, my invention contemplates a reversible, fullycompensated brake organization involving two shoes pivotally mountedwithin a drum and differentially actuated for engagement with the drum.Depending on the direction of drum rotation, one of the shoes will tendto self-lock, and I employ novel mechanical-linkage and leverage meansinterconnecting the shoes and including 'a frame-based (i.e.non-rotatable) reference-for assuring not only that self-locking will beavoided, but also for assuring even distribution of braking effort forthe two shoes. The mechanism is so devised that for a rotating drum, theself-locking tendency of the primary shoe'will *drive the secondary shoesostrongly against the framereferenced linkage that the primary shoewill be with drawn, and actual self-locking cannot occur, the net resultbeing even braking effort by both shoes.

According to one preferred embodiment of the invention, the actuatorsand supports for the shoes are so arranged that the different reactionpressures which occur on the primary and secondary shoes generatemotions in the shoes which are impressed on a mechanical linkagecross-connected between the shoes to stress the primary shoe againstself-locking to afford a smooth and nearly constant braking action. Tothis end, the upper ends of the two shoes are coupled through abidirectional motionmodifying and motion-reversing linkage, such as apantograph lever system, whereby shoe movement deriving from drumreaction thereon causes one of the shoes to be urged in a direction awayfrom the drum. In one a; rangement of the invention, a simple andrelatively low pressure hydraulic actuator system is integrated with thehigh stress mechanical system by housing the mechanical parts in thehydraulic medium and coupling them directly to the hydraulic pistons.

In another preferred form of the invention a pair' of levers arepivotally connected at spaced apart points'to'a brake plate. Theselevers are connected, preferably by pin and slot pivotal connections, toa pair of brake shoes which are urged by said levers into engagementwith the usual brake drum. The shoes are preferably normally urgedtoward each other by spring means in the usual fashion and are linkedtogether by a compression link secured thereto, the effective distanceof said link from the center of the drum being not more than about 0.5the radius of the drum. The levers are rocked by hydraulic or mechanicalmeans, which permit the same to float, and such action of the leversurges the shoes into engagement with the brake drum. There is adifferential action on the two shoes, due to the direction of rotationofthe drum and the friction between the shoes and the drum. The shoes areso interconnected, preferably through the levers and other connectingparts in such a way that one of the shoes has applied thereto'a forcedirected inwardly away from the drum upon the application of a highreaction pressure between the drum and the other shoe. In such manner,the brake is rendered regular in its action and responds properly to theoperator pressure, tending to rock the links on their pivots. In one ofthe forms shown",

the connection between shoes preferably consists of extension lever armson the levers extending across one to the other so that each extensionarmhas'contact with the lever action upon the other lever. Thus,rotation of one lever in one direction tends to rotate the other lever,due to the engagement of the extension lever arm and the abutment whichit contacts.

The above and other features and objects of the present invention willbe apparent to those skilled in the art by having reference to thefollowing specification describing a preferred embodiment of theinvention and taken in conjunction with the accompanying drawings, ofwhich:

Fig. 1 is a view in side elevation of the basic parts of a brakeassembly as viewed along the axis of the drum from the outside;

Fig. 2 is -a fragmentary view in enlarged scale and'in horizontalsection, taken substantially in'the plane of the line 2-2 of Fig. 1,looking in the direction of the arrows;

Fig. 2A is a fragmentary view of a portion of Fig, 2.

'angular movement. nectcd by a pair of retracting or tension springs 23and [24 disposed on opposite sides of the axis of drum rotation andnormally holding the shoes 11 and 12 out of engager 3 plane of the line55 of Fig. 4, looking in the direction of the arrows; and

Fig. 6 is a sectional view, taken substantially in the plane of the line6-6 of Fig. 4, looking in the. direction of the arrows.

Referring to the drawings and to Fig. I particularly, the invention isillustrated as embodied in a brake system including a cylindrical drum10 adapted to rotate, for example, with the wheel (not shown) of avehicle and a pair of segment shaped shoes 11 and 12 suspended, in amanner to be described below, within the drum. The brake shoes, carryingfriction surfaces or linings 11a and 12a, are movable in theirsuspension so that they can be forced into frictional engagement withthe inside wall of the drum 10 by means of an actuator indicatedgenerally by the numeral 13.

The actuator 13, in one arrangement of the invention selected forpurposes of illustration, includes a hydraulic cylinder 14 preferablyfixed to a back plate 14' and within which are mounted a pair of pistons15 and 16 (Figs. 2 and 3) carrying sealing rings 15a and 16arespectively. The pistons 15 and 16 are joined to webs 17 and 18respectively of the brake shoes 11 and 12 by couplings 19 and 20. Thecouplings between the webs and the pistons afford limited freedom ofvertical movement, this through the agency of vertical slide channels19a and 20a within which slide members 19b and20b are slidably received,as with a dovetail engagement. The shoes can, therefore, shiftcircumferentially in their floating mount while in engagement with thedrum, the cylinder 14 remaining fixed. Flexible sealing leaves 15b and16b protect the outer ends of the pistons 15 and 16, as well as thecylinder ends from dirt. The slide members 19b and 20b are in turnpivotally connected to the brake shoe webs. The two pistons are adaptedto be driven apart or outwardly toward .their respective shoes byhydraulic pressure introduced from a suitable source such as a mastercylinder through a hydraulic inlet conduit 21 and ducts 21a to thecenter portion of the cylinder 14 between the two pistons.

The lower portions of the shoes 11 and 12 are coupled through anadjustable compression link 22 disposed on the opposite side of the axisof rotation of the drum 10 from the actuator 13, the link 22 beingcoupled to the respective webs 17 and 18 to afford limited freedom forThe webs 17 and 18 are also conment' with the drum. It will beunderstood, therefore, that'ithe shoes 11 and 12 are supported withinthe drum with certain freedom of movement both radially andcircumferentially of the drum in what is commonly termed a floating andself-centering mounting. In the illustrated [arrangement the effectiveline of the compression link 22 is located approximately 0.275 theradius of the drum measured from the center of rotation of the drum.This contributes to self-locking action in the brake system which willbe discussed more fully below.

If desired, an emergency mechanical brake-actuating linkage can beprovided in the form of an emergency brake cable 25 connected to thefree end of a swinging lever 26 pivotally connected at 27 to the web 18of the brake shoe 12. A tie link 28 is pivotally connected at 29 tothelever 26 and is joined at its other end to the web :17 of the shoe 11through a slide coupling including a pin '30 on the web 17 and a slottedtip 31 on the end of the tie link 28. When the cable 25 is pulled to theleft, as viewed in Fig. '1, the brake shoes are driven apart and intoengagement with the drum 10 by the action of the artic 'ula'ted tie link28 and swinging lever 26.

'In the course of normal operation of a brake system of the typedescribed above, when the brake shoes 11 and -12 are urged intofrictional engagement with the rotating drum 10 by the actuator 13, thereaction pressures of the drum on the two brake shoes differ. .Assumingthe drum 10 is rotating in a counterclockwise direction, as indicated bythe arrow 32, the shoe'll becomes the primary shoe and the shoe 12 thesecondary shoe. The two shoes and the compression link 22, as a coupledsystem, will be urged in a counterclockwise direction around the axis ofdrum rotation.

The two shoes will not, in the normal course of operation, apply equalrestraining or braking forces to the drum and the ratio of the forces aswell as the nature of the brake operation will vary with such parametersas the coefiicient of friction between the working surfaces, the workingsurface areas, the spacing of the compression link 22 from the drumcenter, the hydraulic pressure in the cylinder 14. When the brake is inoperation, it is possible for the shoes to attain a state of equilibriumin which there is either a zero opening force or a negative force, i.e.a force directed to the right at the upper end of the primary shoe 11,as viewed in Fig. 1. The brake system is then said to be self-locking inthat it cannot be released by the normal controlling means.Conventionally, brake systems are designed so that under no conditionswill they become self-locking.

In accordance with the applicants invention as disclosed in his saidcopending application, Serial No. 651,- 477, countermeasures can bebuilt into a highly efficient brake system of the self-locking type toachieve a high degree of regularity of braking action and to obviate theself-locking action. The present invention achieves these ends by meansof a positively acting mechanical linkage, seen particularly in Figs. 2and 3.

The inner ends of the pistons 15 and 16 are coupled to each otherthrough a linkage indicated generally by the numeral 32, the linkageoperating to afford limited freedom of relative movement between thepistons, to amplify inward motion of either piston beyond the limits offreedom, and to reverse the direction of the amplified motion. Thelinkage includes a first system of articulated levers 33, 34, 35, 36, 37and a second system of articulated levers 38, 39, 40, 41, and 42 coupledat their respective ends to the pistons 15 and 16 and themselvespivotally linked at their crossing points by pivot shafts 43, 44, 45,46, and 47 to form a so-called pantograph or lazy tongs type linkage inwhich the pivot shafts 43 and 47 are coupled by means of lugs 48 and 51to the pistons 16 and 15. As seen in Fig. 3, the linkage can bestrengthened by stacking duplicate lever systems on common pivot axes,three stacked systems 32-1, 32-2, and 32-3 being used in the illustratedarrangement. I

The pivot shafts 44 and 46 are extended at either end to engage, whenthe pant'ograph linkage is actuated in a manner to be described,abutment pieces 49 and 50, secured as by bolts 49a and 50a to the innerwall of the cylinder 14. Each abutment piece, 49 and 50, includes a pairof abutment points 49b and 490 and 50b and 50c (Fig. 3) which arepreferably concavely rounded to reas between the two pistons.

In operation, assuming the drum 10 is rotating in a counterclockwisedirection, the hydraulic fluid within the cylinder and between thepistons is subjected to pressure from the master cylinder to drive thetwo pistons outwardly and away from each other to press the shoes 11 and12 against the drum. The reaction pressure of the drum on the shoesurges the shoes, as a coupled system, in a counterclockwise direction.The secondary shoe will be driven, through a combination of its own drumreaction pressure and the reaction pressure of the primary 51106,. astransmitted through the compression link 22 against its actuating piston16, driving it inward against the fluid pressure of the hydraulicsystem. The slide coupling 20'aallows the shoe 12 to move or shift withthe drum while transmitting a major component of its motion in adirection axial with respect to the hydraulic cylinder 14.

During the braking action the shaft 46' of the lever linkage 32 will beseparated from its abutments 49b and 50b, as best seen in Fig. 2A., Thepivot shaft 44, however, will havere-engaged its abutments 49c and 50c,whereupon an axially inward force will be applied by the linkage to thepiston 15. This force will tend to urge the shoe 11 away from the drumand self-locking of the brake system will not occur. The illustratedlinkage affords, in addition to a reversal of force direction, amagnification of motion and a step down in the ratio of applied forcesas between the pistons 16 and 15. There results a compensating force onthe primary shoe 11, which, when the force ratios are properly selectedin relation to the other brake system parameters (including theposition, radiallyspeaking, of the compression link 22), will put thebrake system in balance, preventing possible self-locking andestablishing regular braking action.

It will be observed that the linkage 32 is bidirectional and that asimilar action will occur when, for example, the drum is rotating in aclockwise direction-to render the shoe 12 primary and the shoe 11secondary. Also, the compensating action forces are applied axially ofthe pistons and, in the illustrated arrangement, the mechanicaladvantages are equal for both directions of linkage operation. Moreover,the linkage in the illustrated embodiment, being fully enveloped in thehydraulic medium, is fully protected against damage, includingcorrosi-on.

The invention illustrated at Figs. 1-3 can take various other forms andarrangements to accommodate a wide range of brake designs. and forcemultiplication ratio can be changed for either the counterclockwise orclockwise directions of drum rotation, on for both directions. Difierentratios for both directions can be attained, in accordance with wellknown'lever mechanics by changing the lengths of the lever arms of thelinkage 32 between pivot points. Also, the placement of the abutmentlocations can be changed to change the multiplication factors, and theabutments need not be placed symmetrically, but can favor one directionof'motion over the other. Thus the abutments 490-500 can be placedadjacent the pivot shaft 45 and that shaft elongated similarly to theshaft 44. Moreover, the abutments can be placed oh the axis of thepistons as viewed in Fig. 3 The invention can also be applied to a fullymechanical brake system by, for example, utilizing mechanical spreadersto urge the pistons or members 15 and 16 outwardly or, alternatively thelinkage 32 can be separated entirely from the basic brake actuatingmechanism.

Referring now to Figs. 4-6, there is shown a brake system having adifferent mechanical arrangement for accomplishing generally similarresults. The system includes a usual brake plate 52 of desired properdesign, and on this brake plate are mounted two levers 53-54. Theselevers are pivotally mounted and, in the preferred form, are mounted onpins 55-56 rigidly secured to the brake plate or back plate 52 as bymeans of nuts 57 as shown in Fig. 5. The levers may be held on the pinsas by means of the washers and splitwire rings 58.

The levers are interconnected in such a way that rocking one of thelevers about its pivot in one direction tends to rotatethe other leveraboutits pivot in one direction. In the form illustrated, thelcver.54has an auxiliary plate or lever part 5?) secured thereto andproviding an extension lever arm 60, which has a lever end 61, as willbeclear. The other lever 53 preferably has a lever plate Thus, forexample, the motion I 6 62 has an abutment surface 63 for engagement bythe lever end 61 on the. lever 60, carried by the lever 54. The lever 53has an extension lever arm 64, having an end65, for engagement with anabutment surface 66 on the other lever 54. It will be seen that thetwoextension levers 60-64 are in different planes so that they may crosseach other, as shown particularly in Fig. 4. The lever 54 and itsattached auxiliary extension lever'pl'ate 59 may be separated as bymeans of a spacer plate 67 or the like, while the lever 53 and itsauxiliary extension lever plate 62 may be spaced by a spacer 68. The twolevers at the bottom ends are connected to gether so as to rock the sameabout their pivots by any suitable means, but in the form shown there isa fluid pressure cylinder 69 and a piston rod 70 having a piston in thecylinder 69; cylinder 69 and rod 70 are connected only to the twolevers-53-54' and may therefore float with respect to the back plate 52.Thus, upon the application of pressure fluid through the connection 70'(Fig. 6) to the piston, the two lower ends of the levers will be drawntoward each other. l

The upper ends of the levers 5354 are pivotally connected to a pair ofbrake shoes 71-72. In the form shown, thme' shoes are segmental inshape.The shoe 71 has friction brake surfaces 73--74 while the brake shoe 72has a friction band 75. It might be here stated that the two shoes arenormally urgedtoward each other and out of'eugagement with the brakedrum 76 by tension springs 77-78, as will be understood. The pivotal connection between the upper ends of the levers 53--54- and the respectiveshoes 71-72 may be in the form of pivotal pin and slot connections. Inthe form illustrated, each of the levers, for example the lever 53 andits auxiliary lever or plate 62, carry a pin 79 betweenthem and betweenthe two lever parts the pin rotatably carries a hear ing sleeve 80. Theother lever 54 and its auxiliary lever or plate 59 carry a similar pin81 and a bearing sleeve 82, both corresponding to the same parts shownparticu larly in Fig. 5. The web of the brake shoe 71 cooperates withthe sleeve 80, and in the form shown the shoe web has an elongated notchor slot 83 extending into the same so as to provide a book 83', which ineffect hooks over the bearing sleeve 79. Thus, the'shoe 71 is bothpivotally and slidably connected to the lever 53. The shoe 72 has asimilar slot or notch 84 defining a, hook, which hooks over the bearingsleeve 82 the same as in the other case. Thus, rocking of the levers53-54 will rock the shoes 71--72, as will be understood. The shoes arelinked together by a compression link 85, which is adjustable as tolength as by means of an adjustment screw means 86. The effective lineof the compression link 85, that is, the line joining the two points ofconnection between the link and the shoes, is preferably not distantfrom the center of the drum more than about. 0.5 the radius of the drum,and in the form shown in Fig. 4 this distance is roughly 0.3 the radiusof the drum.

The operation is substantially as follows: The lower ends of the levers,which are fioatingly mounted on the back plate, just as the shoes arefioatingly mounted within'the drum, are drawn together by means of thepiston and cylinder means 6970 or other'means for coupling and drawingthe levers together. By drawing the lever ends toward each other, thelevers are rocked about their pivots 5556 and through the pins 79-81 andthe bearing sleeves -82 within the hook slots of the shoe webs, theshoes are moved out into engagement with the inside of the drum 76. Theelfect'of the rotating drum is different as between the two shoes. Theshoe 71, which is considered the primary shoe when'the drum rotatescounterclockwise, that is, in the direction of the arrow, and the shoe72 are driven in acounterclockwise direction with drum. This action oftherotating'drum on the two shoes acts through the pins'79'-81 and their62'securedthereto, as by means of rivets, andthis plate 7 bearingsleeves 86-82 so as to tend'to" rock bothlevers in a counterclockwisedirection. Since the shoes are floating and the levers are alsofloating, the ,drum reaction on the shoe 72 tends to increase, and anyincrease in that pressure is transmitted to the pin 81, which in turntends-to rock about its pivot 56. Such rocking of the lever 54 in acounterclockwise direction about its pivot 56 acts through the lever arm60 and extension lever end 61 to tend to rotatethe lever 53 in aclockwise direction. Now the forces acting on the lever 53 essen tiallyare three in number: 1, the initial force acting through the piston rod70 tending to rotate the lever 53 in a counterclockwise direction; 2,the force of the extension lever arm 60 tending to rotate the lever 53in a clockwise direction; and 3, the reaction pressure of the drum onthe shoe 71, which acting through the pin 79 may be in a clockwise orcounterclockwise direction, so as to maintain the lever 53 inequilibrium. It should be noted that the forces acting on the shoe 71are the forces transmitted through the pin 79, the reaction pressurethrough the compression link 85, and the drum reaction pressure to theright on the shoe 71. It may be said that the position of the resultantdrum reaction pressure on the shoe depends largely upon the coefiicientof friction between the shoe 71 and the drum. With a normal or highcoefiicient of friction, the resultant drum reaction pressure on theshoe 71 acts to the right in a position below the line of the link 85.With that reaction pressure toward the right and the link reactionpressure toward the left and at a somewhat lower value, the pressure onthe pin 79 must be to the right to cause the shoe 71 to remain inequilibrium and a self-locking situation exists. This effect iscounteracted by the lever system. Thus, depending upon the coefficientof friction between the shoe 71 and the drum, the forces acting on thelever 53 at the pin 79 may be clockwise or counter clockwise, asindicated above. Thus, the shoe 71 has a tendency to move to maintainthe braking action uniform and not irregular as with the prior artdevices of the same general character.

- If the drum rotation should he in the opposite direction, the shoe 72would become the primary shoe, and the shoe 71 would be the secondaryshoe, and the actions would be reversed fromthat heretofore described.Therefore, no matter which way the drum rotates, the action of the brakewill be substantially the same because the action of one shoe when thedrum rotates in one direction will be the action of the other shoe whenthe drum rotates in the opposite direction.

It will be seen, then, that I have provided a brake which will beregular in its action and operate more smoothly and in accordance withthe operators foot pressure than prior-art brakes of the same generalcharacter. Moreover, the brake can be operated under conditions whichwould normally result in self-locking.

While the invention has been described in considerable detail and twopreferred forms illustrated, it is to be understood that various changesand modifications may be made within the scope of the invention asdefined in the appended claims.

I claim:

1. In a brake, a brake plate, a pair of brake shoes to engage arotatable brake drum, actuating means for urging said shoes intoengagement with the drum, and connecting means including reversiblysymmetrical levers between said shoes having a fulcrum reaction againstsaid brake plate and directly acting upon one of said shoes-in adirection away from said drum upon the application of a reactionpressure between the drum and the other of said shoes for one directionof rotation of said drum.

2. In a brake, a brake plate, a pair of brake shoes to engage arotatable drum, said shoes being linked together by an abutment link,the efiective line of said link being spaced from the drum center notmore than about 0.5 the radius of the drum, actuating for urging saidbrake shoes into engagement with the drum, and means includingreversibly symmetrical levers between said shoes having fulcrum reactionwith reference to said brake plate and directly acting upon one of saidshoes in a direction away from said drum upon the application of a highreaction force of the drum on the other of said shoes for one directionof rotation of said drum.

3. In a brake having a drum and at least a pair of complementary shoesfloatiugly mounted in the drum for movement relatively to each other andto the drum, actuating means to spread the shoes to frictionally engagethe drum, a brake plate, first coupling means joining the shoes on oneside of the axis of the drum to transmit forces therebetween, and secondcoupling means joining the shoes on the other side of the axis of thedrum in cluding reversibly symmetrical motion reversing linkagereferenced to said plate and responsive to movement of one shoe in thegeneral direction of drum rotation to directly retract the other shoeaway from the drum.

4. A brake as set forth in claim 3, said motion reversing linkageincluding motion amplifying lever means.

5. In a brake having a drum and at least a pair of complementary shoesfioatingly mounted in the drum for movement relatively to each other andto the drum, actuating means to spread the shoes to frictionally engagethe drum, a brake plate, first coupling means joining the shoes on oneside of the axis of the drum to transmit forces therebetween, and secondcoupling means joining the shoes on the other side of the axis of thedrum including motion-reversing linkage referenced to said plate andresponsive to movement of one shoe in the general direction of drumrotation to directly retract the other shoe away from the drum, saidactuating means to spread the shoes comprising a hydraulic cylinderhaving a pair of pistons connected respectively to the shoes,saidrnotioureversing linkage being mounted within the cylinder andcoupled to the pistons therein.

6. A brake, comprising a drum and a pair of complementary shoesfloatingly mounted within said drum for movement relatively to eachother and to the drum, actuating means to spread said shoes forfrictional engagement with said drum, a brake plate, first couplingmeans connecting said shoes on one side of the drum axis to transmitforces therebetween, second coupling means connecting said shoes on theother side of said drum axis and including motion-reversing linkagereferenced to said plate and responsive to movement of one shoe in thegeneral direction of drum rotation to urge the other shoe away from saiddrum, said motion-reversing linkage comprising a series of articulatedlever arms pivotally connected as a pantograph linkage having opposedends, one of said ends being connected to one shoe and the other of saidends being connected to the other shoe, the reference to said platecomprising'abutment means coacting between said plate and a part of thepantograph linkage intermediate said ends, whereby both ends of thelinkage can move outwardly, with inward movement of either end of thelinkage establishing engagement with said abutment means to urge theopposite end inwardly.

7. In a brake having a drum and at least a pair of complementary shoesto frictionally engage the drum, means to support the shoes for radialand circumferential floating movement within the drum includingcompression link means joining adjacent ends of the shoes on ancffective line which is spaced radially from the axis of the drum, andactuating means between the other adjacent ends of the shoes on theother side of the axis of the drum from the compression link means tospread the shoeslo establish frictional engagement with the drum, andreversibly symmetrical motion reversing linkage on said other side ofthe drum axis and coupling the shoes, said linkage being responsive tomovement of one shoe in the direction of drum rotation and directlyacting upon the other shoe in a direction away from the drum at saidother end of the shoe.

8. In a brake having a drum and at least a pair of complementary shoesto frictionally engage the drum, means to support the shoes for radialand circumferential floating movement within the drum includingcompression link means joining adjacent ends of the shoes on aneffective line which is spaced radially from the axis of the drum,

and actuating means between the other adjacent ends of v the shoes onthe other side of the axis of the drum from the compression link meansto spread the shoes to establish frictional engagement with the drum,and motionreversing linkage on said other side of the drum axis andcoupling the shoes, said linkage being responsive to movement of oneshoe in the direction of drum rotation and directly urging the othershoe in a direction away from the drum at said other end of the shoe,said actuating means comprising fixed hydraulic cylinder means having apair ofpistons coupled respectively to said shoes, said motion-reversinglinkage being connected between said pistons.

9. A brake as set forth in claim 7, said motion reversing linkagecomprising a system of articulated lever arms pivotally linked togetheras a pantograph linkage having opposed ends, one of said ends beingconnected to one shoe and the other of said ends being connected to theother shoe, and fixed abutment means intermediate the ends of thelinkage to be engaged thereby to arrest motion of the pantograph linkagebetween its ends, whereby the two ends of the linkage are urged towardeach other as a function of the abutment position relative to the lengthof the pantograph.

10. A brake as set forth in claim 7, said compression link being spacedfrom the drum center not more than about 0.5 the radius of the drum,whereby a self-locking brake system is established, said motionreversing linkage preventing actual locking thereof.

11. A brake as set forth in claim 9, said abutment means including afirst abutment to be engaged upon inward compression movement from oneend of the pantograph linkage and a second abutment to be engaged uponinward compression movement from the other end of the linkage.

12. Brake actuating means for a pair of brake shoes which operate inconjunction with a brake drum, comprising a pair of movable membersadapted to be connected respectively to the pair of brake shoes, a brakeplate, actuating means to move the members in a direction to urge theshoes outwardly, and reversibly symmetrical linkage means coupled to thetwo members and including a fulcrum abutment with a part of said plateand responsive to movement of at least one member to directly act uponthe other member in a direction tending to urge its corresponding shoeaway from the drum.

13. Brake actuating means comprising a brake plate, hydraulic cylindermeans, a pair of relatively movable pistons in the cylinder, and motionreversing linkage coupling the pistons and including a fulcrum abutmentwith a part fixed with respect to said plate and responsive to inwardmovement of one piston to urge the other piston in an inward direction.

References Cited in the file of this patent UNITED STATES PATENTS1,889,093 Gunn Nov. 29, 1932 2,140,385 Hunt et a1 Dec. 13, 19382,375,392 Stelzer May 8, 1945 2,382,268 Stelzer Aug. 14, 1945 2,657,768Hunyady Nov. 3, 1953

